Methods of electrolytic grinding and eroding



Se t. 3, 1957 N. w. THIBAULT ET AL r 2,395,197

METHODS OF ELECTROLYTIC GRINDING AND ERODING Filed Nov. 7, 1955 5 Sheets-Sheet l A ATTORNEY Sept. 3, 1957 N. w. THIBAULT ET AL METHODS OF ELECTROLYTIC GRINDING AND ERODING Filed Nov. 7, 1955 5 Sheets-Sheet 2 .fNVENTORj lVEWMAN W 7703mm?- 550E56- 550445700/(374 AF? ATTQZNE 7 N. w. THIBAULT ET AL 2,805,197

METHODS OF ELECTROLYTICGRINDING,AND ERODING Sept. 3, 1957 Filed Nov. '7, 1955 5 Sheets-Sheet 4 I INVENTORS.

NEWMAN W TH/BAULT Gar/e5 E. fiu/ws ackj'r l lulaldl ilni1lltam0 United States Patent 01 METHODS OFELECTROLYTIC GRINDING AND ERODING Newman W. Thibanlt, Worcester, Mass., and George E. Comstock 311., Huntington, N. Y., assign-ors to Norton Company, Worcester, Mass, a corporation of Massachusetts Application November 7, 1955, Serial No. 545,434 16 Claims. (Cl. 204-143) The invention relates to method of electrolytic grinding and eroding. This application is a continuation-in-part of our copending applications Serial No. 326,974, filed December 19, 1952, now abandoned, and Serial No. 342,065, filed March 13, 1953, now abandoned.

One object of the invention is to provide a method of the type indicated to prevent channeling of the electric current. Another object is to provide a non-arcing flow of direct current between a metallic conducting workpiece anode and the wheel which is the tool cathode. Another object is electrolytically to erode workpieces without setting up gradients between parts of the wheel cathode which etch the wheel.

Another object is to provide a method of electrolytically eroding workpieces capable of removing stock at higher.

rates than heretofore practicable. Another objectis to remove any metal load upon the wheel as fast as it is. formed, i. e. to keep the wheel free from metalload; Another object is to provide a method of shapinghard.

materials such as the hard carbides especially without the use of expensive diamond containingwheels or the like. Another object is to provide a method of shaping workpieces which can be mainly or entirely by electrolytic work erosion but which optionally may involve some 56 (Figure '1 right hand side) and when the desired abrasive action as well.

Other objects will be in part obvious or in part pointed out hereinafter. I

In the accompanying drawings illustrating many mechanical and electrical features of the invention:

Figure l is a front elevation of the electrolytic eroding apparatus,

Figure 2 is a side elevation thereof,

Figure 3 is a sectional view on an enlarged scale taken on the line 3-3 of Figure 2,

Figure 4 is a fragmentary horizontal sectional view on an enlarged scaie of the tool cathode wheel and workpiece holder,

Figure 5 is a fragmentary side elevation on an enlarged scale illustrating the work holder clamp,

Figure 6 is a front elevation of the wheel guard cover and associated parts,

Figure 7 is a fragmentary view partly in side elevation and partly in section of the cathode and its mounting for removing load upon the wheel,

Figure 8 is a wiring diagram,

Figures 9, l0 and 11 are alternate wiring diagrams,

Figure 12 is a graph illustrating the results achieved by grinding and work erosion with a recrystallized silicon carbide wheel and a boron carbide wheel compared with a standard type of metal bonded diamond wheel.

Referring first to Figures 1 and 2, the machine has a work table 11 supported on ways 12 and 13 provided 'on a cross slide 44 mounted on the machine base 15. The ways for the cross slide 14 are not shown but may be conventional and reference may be had to U. S. Patent No. 2,101,787 for many of the constructional features of this illustrative embodiment of the invention. A screw shaft 16 passes through a nut 17 which is aflixed to the base while the rear end of the screw shaft 16 is jourice 2 nalled in a two way thrust bearing 18 which is attached to a rearwardly projecting portion 19 of the cross slide 14; rotation of the screw shaft 16 as by means of a hand wheel 20 therefore moves the cross slide 14 forwardly or rearwardly of the machine.

The table 11 can be reciprocated upon the cross slide 14 by the fluid pressure mechanism described in the aforesaid Patent No. 2,101,787 and in Figure l we show a piston rod 21 connected to a bracket 22 which is attached to a laterally extending portion 23 of the table 11. We need not illustrate or describe the hydraulic mechanism of the aforesaid Patent 2,101,787 which is controlled by the table logs 24 and 25 which alternately engage an interposed reversing lever 26 mounted on a shaft 27 projecting from the front of the cross slide 14. We do illustrate herein, however, manually operated apparatus for moving orreciprocating the table 11 comprising a rack 4 30 secured to the under side of the table 11 and engaged by a gear 31 which is secured to a larger gear 32 which meshes with a smaller gear 33 secured to a shaft 34 on the front end of which is a hand wheel 35.

The base 15 supports a vertically extending split column 41 which is provided with a cylindrical bore, not shown,

receiving-a cylindrical post 43. Adjustably secured in any desired angular position to the top of the post 43 by meansv of bolts..44 extending .through angular slots,

. not shown, is a wheel head 45 journalling a cathode wheel wheels has a resistivitymany times, usually a thousand spindle 46 which projects both forwardly and rearwardly of vthe wheel head 45. Afiixed to the rear of the spindle 46;is: aipulley 47 driven by-a belt 48 from a pulley 49 on the armature shaft 50 of a motor 51 which is secured by means of bolts 52 to a table 53 having slots 54 receiving bolts 55 screwed into =the wheel head 45; thus the motor'Sl, is secui'ed in verticallyadjustable position to the wheel head 46. The cylindrical post can be raised and lowered by mechanism described in the aforesaid Patent No. 2,101,787 which is operated by a hand wheel adjustment is obtained the post 43 can be securely locked by means of a screw 57 extending across the gap in the split column 41.

Referring now to Figure 4 as well as to Figure 2, on the front of the spindle 46 is a nut 60 which holds a flanged sleeve 61 having a tapered bore on the front end of the spindle 46 which has a conical portion 63 engaging the tapered bore of the sleeve 61. A so-called cup shaped wheel 65 is secured to the flanged sleeve 61 by means of a spanner nut. 66 which engages an exteriorly threaded hollow cylindrical portion 67 integral with and extending forwardly from the sleeve 61, the usual hole in the center of the back of the eroding wheel 65 being of a size nicely to fit the portion 67 thus to center the eroding wheel 65 on the spindle 46 while the nut 66 clamps the back against a soft metal washer 68 and the flange of the sleeve 61. Another soft metal washer 69 is interposed between the nut 66 and the back of the wheel 65; thus the wheel 65 can be securely clamped to the sleeve 61 without danger of cracking the wheel.

The cathode wheel 65, according to one embodiment of the present invention, which is sometimes referred to as a grinding wheel .although usually its abrasive action is minor compared with the stock removal by the electrolytic process, is a recrystallized silicon carbide wheel. This is quite different from a vitrified bonded wheel. One of the widely used types of grinding wheels is made of silicon carbide bonded with ceramic material such as clay or a mixture of clay and other ceramic material or materials. Wheels of' this nature are well known and have been used for years to grind various metals and also carbides. The material of all of such wheels is virtually electrically non-conductive; at least the most conductive of. such times, as much as a piece of recrystallized silicon carbide. At room temperature the resistivity of recrystallized silicon carbide ranges from a large fraction of an ohm centimeter to 50 ohms centimeter. On the other hand even the smallest of these resistivities is tremendous compared with the resistivity of the typical metal bonded diamond wheel heretofore proposed for electrolytic grinding. Such wheels, usually bonded with metal the base of which is copper, have resistivities of the order of one hundred thousandth of an ohm centimeter.

Recrystallized silicon carbide was described by Francis A. J. Fitzgerald in U. S. Letters Patent No. 650,234, patented May 22, 1900. It is made by heating cryst'als of silicon carbide in proximity to each other to the tempera;- ture of recrystallization which means the temperature of further crystal growth. s

For many years recrystallized silicon carbide has been formed into rods for use as electrical resistors in high temperature furnaces. This has been thechief use of this material and so far as we are aware heretofore no grinding wheels of recrystallized silicon carbide have been made or used. I

For the manufacture of wheels aecording to the inven; tion, we may proceed as follows: We make a mixture pr silicon carbide grains and sodium silicate solution: For example, 45% by weight of No. 20 grit size silico earbide and 55% by weight of No. 100 grit size sili'oncarbide are thoroughly mixed. Then for each pound of this mixtureof silicon carbide grain, we provide 14 'clibic centimeters of 3 to 1 sodium silicate solution, that s to say 3 parts of water to 1 part of sodium silicateby weight. Making a thorough mixture of the silicon bide grain and the sodium silicate solution, we mold the mixture into the desired shape under pressure, for ex ample in an hydraulic press, dry the pressed wheel for the removal of all but about 2% of the water, andthen fire the pressed and dried wheel to a temperature to recrystallize the silicon carbide. A good recrystallizing temperature is 2500 C. and we can use a graphite tube furnace or an induction furnace having graphite crucible to hold the wheel or wheels being recrystallized. A satisfactory furnace is illustrated in Raymond R. Ridgways U. S. Letters Patent No. 2,125,588. This procedure produces recrystallized silicon carbide wheels having a cross-bending modulus of rupture in the range of 5,000 to 7,000 pounds per square inch and a room temperature resistivity that may vary from as high as to 50 ohms centimeter if carefully selected crystals of green silicon carbide were used, to a value several orders of magnitude smaller if black or gray grain were used. It may be noted at this point that the unit ohm centimeter is exemplified in a cubic centimeter since resistivity increases proportionally to the length and decreases proportionally to the area of cross-section. This same unit used to be called ohm per cubic centimeter. I

The cathode eroding wheel 65, according to another embodiment of the present invention, is a molded piece of boron carbide. Boron carbide is very hard, having a hardness well above 9 on Mohs scale and exceeding even silicon carbide and tungsten carbidein hardness. In fact it is supposed to be the hardest known substance apart from diamond. I

Boron carbide material of the nature indicated can be made in accordance with U. S. Patent No. 2,155,682 to Raymond R. Ridgway and can be molded as set forth in the patent to R. R. Ridgway and B. L. Bailey, No. 2,027,786. A superior molding furnace for this purpose is fully described in U. S. patent to R. R. Ridgway No. 2,125,588. Millions of pounds of molded boron carbide pieces have been made according to the general procedure set forth in Patent No. 1,897,214 and more particularly described in Patent No. 2,155,682 and in molding apparatus as described in No. 2,125,588. This, therefore, is the material of which the wheel 65 may be composed in accordance with the present invention.

However, it is sometimes preferable to have a boron carbide wheel 65 which is porous, whereas boron carbide articles made as above described have always been dense and non-porous. To make a porous wheel 65 of boron carbide we vary the manufacturing technique by heating the material in its graphite mold without any simultaneous application of pressure, or only a small amount of pressure. Thus the graphite mold is filled with boron carbide grain (using preferably grain of significant size rather than impalpable powder) and the grain is pressed in the mold outside of the furnace toa' pressure of usually about 2500 pounds to the square inch, more or less Then the mold is placed in the furnace and is heated to the sintering temperature which, when no pressure is applied, is from about 2250 C. to 2300 C. We find it advantageous to add a quantity of sodium silicate solution to the boron carbide grain, to function as a temporary bond. About 4% by weight of a solution of one part of sodium silicate to two parts of water of the weight of the boron carbide is a good proportion. The resistivity of molded boron carbide of which the wheel 65 is made ranges from .05 ohm centimeter to 10 ohms n r- Electrolytic so-called grinding is now being done on a commercial scale to shape andsh'arpen tool bits made of cemented carbide (tungsten and/oi: tantalum and/or titanium carbide bonded with cobalt and/0,1 Usually the wheels used are metal bonded diamond wheels although some metal bonded silicon carbide was av b n s T e ra fi' fi ie s? as; is' to say critical diamondrtiaterial been saved but Precautions have hadto be taken to keepthe electrb ytic process from eroding portions of the bitsaway from grinding locils, rounding the edges of tool or eating into the steelportioh's of the tofol to which bits are in many cases welded. Furthermore current strength has hadto be limited so that it wasapparen't that the full capabilities of the electrolytic pro es were not being realized. But by using wheels far-less conductive thin metal bonded wheels, we are able to erode tools and bits withoutexcess precautions and greatly to increase the rate of stock removal. An illustrative workpiece 71 is shown in the drawings; see Figure 4'.

In another embodiment of the inventio'n, we may use a mixtur e 'of silicon carbide and boron carbide. This mixture may be either boron carbide with silicon carbide in solid solution therein or preferably it is boroucarbide with some silicon carbide in'solid solution therein but more and usually a lot more as a separate crystalline phase. Part of this separate crystalline phase may be silicon carbide recrystallized from solution in the boron carbide and part of this silicon carbide phase may be crystals of silicon carbide which were never dissolved iii the boron carbide. The technique of making articles of this kind is fully set forth in U. S. Patent No. 2,109,- 246 to Boyer and Rose patented February 22, 1938, to which reference may be made for a full disclosure of this material and articles made therefrom. Owing to the fact that a satisfactory abrasive action is obtaine'dfrom a two phase material, if some of the silicon carbide is other thanthat dissolved in the boron carbide, dense wheels of this material as taught by the patent to Boyer and Rose may be used. If a single phase material is used, it is preferable that it be porous. The usual rule that the combined action of heat and pressure makes, a nonporous body applies in this case. Boyer and Rose describe' both. A non-porous wheel will electrolytically erode in a highly practical manner and it is chiefly when abrasive (grinding) action is also desired that we prefer that the wheel be porous. Frequently especially when fi'ne finishes are desired stock removal with substantially no abrasive a ction will be satisfactory or even preferred.

For the manufacture of strong articles without flaws the combined action of heat and pressure is preferred. Boyer and Rose point out that boron carbide will dissolve silicon carbide up to a content of about 35% and that at some percentage above this there will be a separate phase of silicon carbide in the article. Therefore for the composite or mixed carbide wheels we prefer to use 35 or more of silicon carbide in order to have a separate phase of silicon carbide present. In this specification all percentages are by weight. The resistivity of this material is substantially the resistivity of the boron carbide and the silicon carbide components thereof and for any proportions of the one to the other a simple engineering calculation will give the resistivity. In any case the resistivity will be within the range desired for carrying out the method of the present invention as each of silicon carbide and boron carbide has a resistivity within that range. To produce well bonded and strong wheels out of the combined carbides, since the boron carbide is acting as a bonding material in these wheels, there should be at least 5% of boron carbide. Wheels made of silicon carbide and having boron carbide less than 5% are considered to be recrystallized silicon carbide wheels, and wheels having only the boron carbide phase even though some silicon carbide is in solid solution therein are considered to be boron carbide wheels, whether the figure of is accurate or not. Therefore we claim as our in-- vention herein the method or process as defined in the generic claims if the wheels are made of boron carbide, or are made of recrystallized silicon carbide, or are made of combinations of the two in any proportions.

Referring now to Figures 1 and 2,;the table 11 which may also be called a carriage, supports aswivel table. 75

which can be set at various angleson'. the table 11 and; clamped in place all in a manner well known in'the grind-- ing machine art. The upper portion of theis'wivel table 75 is inthe form of an elongated dovetail upon whichis.

mounted, for securing in any desired positiontalong' the table 75, a dovetailed cut-out bottom of-a work holding fixture 78 which is secured in such desiredposition by means of a screw 79. By means of a bolt and nut combination 80 extending through spaced parallel upright portions 81 of the fixture 78. a vise base 82 is secured in desired angular position (on a horizontal axis of adjustment) to the fixture 78.

Referring now to Figure 5, a movable vise jaw 83 and a fixed vise jaw 84 cooperate to hold a metal work holding bar 85 with interposed insulating pieces 86 between them and also under the work holding bar 85. Referring to Figure 2, the movable vise jaw 83 is operated by the usual screw 87 through a hole in the end of which extends the usual rod type handle 88 and the screw 87 of course engages a nut or nut portion, not shown, affixed to or integral with the vise base 82. Thus the metal work holding bar 35 is held and clamped by the vise but is electrically insulated therefrom while the vise is secured to the swivel table 75 which in turn is secured to the reciprocable work table or carriage 11 and provision is made for all sorts of adjustments, namely to adjust the position of the work holding bar 85 along the length of the table 11, to adjust the bar 85 angularly on a vertical axis and angularly on a horizontal axis while the hand wheel 56 can be used to raise and lower the eroding wheel 65 relative to the work holding bar 85. Referring to Figure 4, the workpiece 71 is held by the bar 85, as it extends through a hole therein and is secured in place by means of a screw 89.

Referring now to Figure 2, for a continuous power infeed of the cross slide 14 to advance the workpiece 71 at a steady rate toward the annular face 72 of the eroding wheel 65 an electric motor 90 drives a speed reducing mechanism 91 which is secured to the rear end of the rearwardly projecting portion 19, and the speed reducing mechanism 91 drives a sprocket pinion 92 connected by a sprocket chain 93 to a large sprocket gear 94 which is secured to the rear end of the screw shaft 16. In the practice of the invention this power infeed can be used or the workpiece and the eroding wheelcanho advanced, one relative to the other, by manually rotating the hand wheel 20 or any other desired infeed mechanism can be employed, such as the well known intermittent infeed found on many types of grinders. However for conservation of the eroding wheel 65 a continuous infeed such asproduced by the mechanism just described is preferred.

Referring now to Figure 1, we provide a tank 95 containing liquid 96 which in accordance with this invention is an electrolyte. Ordinary salt water can be used, for example clean ocean water can be used although in most places it will probably be more convenient to use a solution. of sodium chloride in water. However it is preferable to inhibit rustingof the machine and of steel parts of the workpiece so far as possible. In most grinding operations the coolant has been water and it has been general practice to introduce a rust inhibitor into the wa ter. Most of these will serve as the electrolyte. Particu- 'lar salts which can be used in water solution to make the electrolyte with rust inhibiting characteristics are sodium nitrite, sodium chromate, potassium nitrite, potassium chromate, the amines of sodium nitrate, sodium and p0- tassium dichromate, sodium and potassium chlorate and sodium and potassium chlorite. Two good formulae are disclosed in British Patent No. 491,023 as follows:

Formula N0. 1

A. concentration in water of:

. Percent Potassium 'chr 22 Potassium dichromate 78 :FormulaNaZ A 1% cor'ic'entration'in water of:

, v Percent Potassium chr t 12 Potassium dichr 86 Sodium chlorat 1 Sodium chlorite l .The above formulae are rust inhibiting formulae and for electrolytic eroding we prefer, in general, about 5% concentrations.

Mounted on the cover plate 97 on the bank 95 is an electric motor 98 which drives a pump 99 the input end of which is connected by means of a pipe 100 to the inside of the tank 95, the open end of the pipe 100 being preferably near the bottom of the tank. The output end of the pump 99 is connected by pipes 101, 102 and 103 to a flexible hose 104 which is connected to a valve 105 on the end of pipe 106 mounted on the wheel guard cover 107a of a wheel guard 107 secured, as shown in Figure 2, by brackets 108 and 109 to the wheel head 45. Referring now more particularly to Figure 6, by connections to be hereinafter described, part of the electrolyte from the pipe 106 is directed to a deformable nozzle 110 which can be bent with the hand to give it the desired curve and when bent into a particular shape will keep that shape despite the flow of liquid therethrough under moderate pressure. This deformable nozzle 110 will be set in such shape as to direct the liquid onto the face of the eroding wheel 65 over the entire locus where it will be contactedor nearly contacted by the work piece 71.

Referring to Figure 1, the liquid which flows onto the workpiece 71 is eventually collected by a large pan 111 whichis built around the top of the table 11; as shown in Figure 2, a spout 112 collects the liquid and allows it to flow into a stationary pan 113 supported by the column 41 and extending for the full length of maximum travel of the spout 112. A return pipe 114 extends from the pan 113 to the tank 95.

Referring now to Figure 3, we provide a pair of brushes in contact with the flanged sleeve 61, and these brushes are held in such contact by means of arms 121 articulated to an insulated block 124 secured by a pin the asaefsj 120 was a cdnnect'idn b'ox' 127 i whieh' the I s 126 are e'niiected to a lead in wire 130, The

articulated arms 1"2 ar'j pnnectd y a spring isiniu's serving to keep epresses I20'in' contact with the 5 flanged sleeve '61. The'c'ou'nectioii box 127 is secured to the block 124. v I

Referring now tdFi'g'urs s and 7, we providea' block or metal 261" which is eatnsde re t wheel 65 for n -"pnai s'aqf retrievin metal load from the face of die when; 'Ih e'jvheel as is alis an-mime to the block zstnth a blbc'li'261 cas a 'reet iigniar parallelepiped which is one offthe siuiplest 'shaaestoqisrm; It is supported by a ante j'vhich cah'b'e stand of comprising a lower pl'af'e 'zj6 z which nests-er 261 is slidable, an upper plate 263 secures by bolts :54- to-rh e w en guard 101 and an integral a art-acting portion shaving a boss 266 a bit pfre' 267 receiving aspr'in'g which urges the block 261 toward the wheel face 72. The spring 268 can be a spring of light tension, adjustable by means of a thumb screw 269, so that the b'lock 261presses very lightly against the face of the'wheel 65. The frame is sFrigthened by bars 270 and 271 secured to thelower 1515232 by s'er'eiris' 2'72 and 'tld 'fhe upper plate 263 by screws "2'73'and'piiis 274 555215 may be used accurately to align the parts. The bars 270and 271 hold the block 261 with a sliding fit thus to keep it in alignment with the wheel face 72.

We further p r qv ide means to deliver liquid electrolyte 9 6 to th e bl ock 261 and between it a nd the wheel face as illustrated in lFignre 6. The pipe 106 leads to a T-union 280 g1nected to a pipe 281 connected to an elbow 282 connected to a pipe 283 tqwhich the deformable nozzle iitris striated; But the T-ilIlidfl 230 also has a nozzle is: c'aiiiiactea thereto delivering liquid an over the face of the wheel 65 where it is running under the block 261. Thejelectr'olyt'e supply pipes and connections can be seciired to jtli'e ester 197a of the wheel guard 107 as by brackets 28 6;,a"nd 287 securing the pipes 281 and 283 to de riva- 107a as show/n in Figure 6.

Referring new to Figure 8, we'utilize a source of direct current electric energy, such as a battery 288 of Edison dens, Referring to Figure 7, screwed into the upper pl 263 is a binding past 289. This binding post 289 is 'eoaiiated, as shown 'in'Figure 8, by a' wire 290 to a variable resistance or rheostat 291 which is connected by a conductor 29 2 to a selecting tap 293 of the battery 288. The positive pole of the battery 288 is connected by a wire 294 to a hand switch 295 which is connected by a wire 296to the workholding bar 85. The wire 130 is con'rie' ed to a variable resistance or rheostat 297 which i's connected by a'conductor 298 to aselecting tap 299. We preferably place an ainmcter 500 in the conductor 298 and preferably we connect a volt meter 301 by a wire 302 to the wirj 296 and by a wire 303 to the wire 130. The block 261 in this illustrative embodiment is not grounded; in Figure 7 we show insulation 305 between th'e' plate 263 and the wheel guard 1.07 and insulation 306 around the screws 264.

It should be understood; however, that the battery 288 is illustrative only inson'iu ch as direct current obtamed by rectifying alternating current can be used, a motor generator set called a coiiverte'r can be used, and other source of direct current electric energy can be used. In other words pulsations of electroniotive force do not interfere with the process provided the peaks are not so great as to cause arcing.

Ill operation, with the switch 295 closed, the workpiece 71 is eroded by the electric current flowing from it to the wheel cathode 65 through the liquid flecfro'lyte' which covers the face of the" wheel 65; There islittlel tendency to build up metal load on the face of the wheel 65 but-any metal load that accumulates" willbe removed by the block 261. It will not be necessary to cause much of the total current to go through the path 290, 291, 292 because, the boron carbide will not collect nietal load to any large extent. However,- onthe other hand it- Willnot be detiirnental to haye' more current than necessary going through the" path 290, 291-, 292 This simply means that machine is very easy to set. The electrolytic action will not erode the boron carbide because it is essentially inert to electrolysis in solutions such as given in Formulae Nos. land 2. As an arbitrary matter the resistances 291 and 297 can be set to take 01f about one tenth of the current through the path 290, 291, 292 thus causing ninetenths to go' through the alternate path 130, 297, 298.

We can start with a wheel 65 the operative face of which has been ground to a flat plane (or otherwise shaped) surface. In view of the hardness of boron carbide, in view of the fact there is no second phase of mate"- rial present (as glassy bond), in view of the fact that the wheel is one integral piece of boron carbide (not i'nyriacls of crushed crystals bonded with vitrified clay), in view of the fact that abradin'g grinding can be avoided without loss of production, in view of the fact that metal load is removed from the fact of the wheel '65 as fast as it is deposited, the wheel 65 will keep its 'shap'e" and originalsurface characteristics for a long time. However, for many practical operations; a surface produced in the inolding operation itself will be accurate'ehough and smooth enough for the work eroding operation so that the expense of grinding the operative surface of the wheel 65' ca'n' be" avoided. Boron carbide is usually ground with diamond grinding wheels.-

The motor can be controlled by a rheostat (not shown) so that it can be run fast and slowly or at any in termediate speed. It can be a direct current motor as di' rect currents motors can be more readily control-led. Also there should be an amineter (not shown) in the circuit energizing the motor 51 which can be an A. C. motor if desired. The operator can set the rate of feed by means of the rhe'ostat for the motor 90 to the rate desired, then watch the ammeter for the motor 51 and reduce the resistance in resistances 291 and 297 sufficiently to keep the motor 51 from overloading.

If at any time the feed of the work to the wheel is at such a high rate as to threaten to fracture the wheel, that danger is made known in advance by the noise level. When the noise level rises more current should be sent through the workpiece 71 (the operator can watch the ammetcr 300) and if the noise level does not drop, the motor 90 should be slowed. The volt meter 301 provides a good check on the resistance 297.

But we contemplate that our invention will in many cases be embodied in simpler machines, such as machines for oif-hand eroding where, instead of mechanical traversing and infeed, the machine has suitable rests and the operator feeds the work to the wheel, connecting the work, howeverr, in the electric circuit as described, and it is best to use the mechanism of Figure 7 although load removal can also be through a block 261 held in the hand but electrically connected and load removal may be intermittent. For example the mechanical parts of the machine might be as shown in U. S. Letters Patent No. 2,375,619 to W. M. Bura. In such machines the cathode eroding wheel can be mounted on a spindle insulated from the bearings and the entire machine can be connected to the positive side of the source of electric energy, which may be grounded for protection of the operator, so that the workpiece, or a fixture holding it, resting upon the metal table of such a machine according to the Blue. patent will be connected to the positive side of the source whereas the cathode eroding wheel is connected to the negative side of the source,

The useof a recrystallized silicon carbide wheel as a cathode wheel in electrolytic stock removal presents decided advantages as compared with the use ofmetal bonded diamond wheels which currently have been preferred. Many of these advantages have already been pointed out. In the first place diamond wheels are expensive while recrystallized silicon carbide wheels are relatively inexpensive. In the second place the supply of diamonds islimited, no great quantity of diamond has yet been synthesized and the ever mounting demand for diamonds has to be met by a relatively few sources.

Thirdly, because the conductivity of silicon carbide is very low relative to copper, arcing is practically eliminated yet heavier currents can be used. Using the copper bonded diamond wheels, when the area of contact between wheeland work became low, the current density became high and arcing frequently occurred. Arcing damages workpiece and wheel alike. It leaves a poor finish on the work.

In the fourth place, greater production can be obtained with a recrystallized silicon carbide wheel than with a metal bonded diamond wheel because it is safe to use a heavier current using the former. In the fifth place, the recrystallized silicon carbide wheel will retain its shape and surface longer than a metal bonded wheel. This makes it possible readily to grind complicated shapes with a formed wheel whereas formed metalbonded diamond wheels have been infrequent because the diamond depth has been limited to one quarter of an inch at most, sometimes onesixteenth of an inchor less. Thediamond bearing portion is too expensive to waste by forming shapes. thereonafter manufacture and shapes-required constantly vary -so that preforming particular shapes is impractical. 4

,Re'crystallized silicon carbide produced herein'ide I scribed. andfwhich can be made in the formiof wheels for use according to this invention is porous throughout and the pores are generally interconnecting. A ground working surface on such a wheel has many plane areas (or they could be curved in some cases) with pores in the form The use of a boron carbide wheel as a cathode wheel in electrolytic stock removal presents decided advantages as compared with the use of metal bonded diamond wheels which currently have been preferred. In fact the boron carbide wheel has substantially all of the advantages pointed out for the recrystallized silicon carbide wheel, except that form shaping of the boron carbide wheel is more difficult than of the recrystallized silicon carbide wheel, but even here the boron carbide wheel has an advantage over the metal bonded diamond wheel.

The combined silicon carbide-boron carbide wheels present substantially the same advantages as the recrys tallized silicon carbide wheels and the boron carbide wheels. The combination is readily moldable and indeed can be hot pressure molded with the apparatus of the Ridgeway Patent No. 2,125,588 previously referred to. In general we believe that recrystallized silicon carbide is preferable to boron carbide from the standpoint of rate of cutting because the silicon carbide is more abrasive. But on the other hand stronger and more uniform wheels can be made by hot pressure molding boron carbide. The combination of the two has to a certain degree the advantages of each.

The voltage drop across the wheel, the resistivity of which has been properly proportioned with respect to the contact areas, electrolyte resistivity, and grinding pres-' sure, may be several times the voltage drop across the electrolyte. Under these conditions it is clear that the Owing resistance of the wheel controls to the larger extent the actual current flow in the circuits. Consequently the resistors 291 and 297 may be made to have small resist ance compared with the wheel, or may be omitted altogether, and the current levels may be adjusted by sliding thevoltage selecting taps 293, 299 on the D. C. voltage source. 1

Attention is invited to the fact that the spanner nut 66, the flange of the sleeve 61 and the interposed washers 68 and 69 constitute metallic clamping means clamped against the wheel and rotating therewith. For many eroding operations something contacting the wheel 65 such as the block 261 cannot carry enough current. But through the soft metal washers 68 and 69 a very heavy load of current can be directed. Also the brushes 120 can carry a very heavy load of current without setting up undetermined resistance which would be the case if the current had to flow through the wheel spindle into the machine bearings through a film of oil.

We have found that sometimes it is better to space the block 261 from the wheel 65 rather than to press it thereagainst by light spring pressure. So therefore as an optional feature we illustrate in Figure 7 a knurled headed screw 310 extending through a plain hole in a boss 311 welded to the bar 270, the screw 310 also extending through a threaded hole in a boss 312 brazed to the block 261. ,By turning the screw 310 the block 261 can be backed away from the wheel 65 and the combination of the screw 310 .and. the spring 268 permits delicate adjustmerit down to'.00l inch.' Of course the greater the gap between block zfljand wheel 65 the greater is the elec- I thafis lias'a' circular periphery, and in a preferred form of theinvention it has. plane annular surface which does the grinding; Also in practice it will usually have a central hole coaxial with its circular periphery. However the cylindrical periphery of a wheel will be the working surface in many applications according to the present invention.

Figure 12 gives actual work erosion results of a recrystallized silicon carbide wheel, of a boron carbide wheel, and of a metal bonded diamond grinding wheel for comparison. The boron carbide wheel was five and a half inches in diameter, as the available molding equipment would not make a larger one. The silicon carbide wheel and the metal bonded diamond wheel were each six inches in diameter. The grinding surface of each wheel was a plane surface of annular shape adjacent the periphery. The metal bonded diamond wheel was a D120-N100M wheel in which D stands for diamond, 120 stands for the grit size, N stands for N grade which means in connection with the M that the bond was 18.6 tin the balance copper, and the stands for the concentration which means 25% by volume diamond. This 18.6 tin balance copper is a widely used formula and in fact probably the most used of any metal bond formula. The results of the test are shown in Figure 12 where the several lines give the graph of erosion or grinding as identified by the legends, As also indicated by the legends the workpiece was pressed against the boron carbide wheel with 18 pounds per square inch, against the recrystallized silicon carbide wheel with 18 pounds per square inch, but against the diamond wheel with pounds per square inch. The diamond grinding wheel was revolved at 1870 R. P. M. and the other two wheels at 1860 R. P. M. The vertical coordinate represents rate of stock removal in grams per minute and the horizontal coordinate represents average current density in amperes per square inch.

All of the tests were made with electrolyte flowing over the face of the wheel and the material being ground was in every case cemented tungsten carbide marketed under thetrademark Kennametal'. The great difierence' between the pressures required for removal of stock withthe=metal bonded diamond wheel and the other two wheels shows that' the boron carbide wheel and the silicon' carbide wheel did little grinding but were removingmetal mostly by electrolytic erosion. We believe theboron carbide wheel was doing practically no grindin'g at all whereas the diamond wheel was mostly grinding-slightly assisted by electrolytic erosion. We believe the silicon carbide wheel was mostly electrolyticallyeroding butwas doing' substantial grinding;- Forcertain finishes it is prefetied to 'erode the work electrolytically and for these 'purposes theboron carbide wheel is sup'erior' to the others.

Thereas'on for using LIO'pQunds-per square inch-- pres: sure against the diamondwheel is inorder to achieve acomparable' rateof stockremoval; Probably the other twowheels we're not strong enough to stand 110 pounds per-"square inch pressure but no such pressure was needed as is shown. Stronger wheels than those we used can of course be made. Much grinding of the hardcarbides is' done olf -h'and. It is agreat strain on an operator'to require him to achieve a pressure of 110- pounds per square inehwhich is another reason whythe process-of the-invention utilizing the'boron carbide and'silicon carbide and boron carbide-silicon carbide wheels is a great" improvement'in the art. In-order'to achieve strictly elec'' trolyti'c work erosion without grinding using themetal bonded diamond wheel thepressure wouldhave to be much below 110- pounds per square inch and then-them'etal bonded diamond wheel would not remove'stock as would either of the two other wheels or a" wheel made of combined boron carbide-silicon carbide.

It will thus be seen that there has been providedby this-inventionmethods of electrolytic grindingand' erodmg in which the various objects'hereinbefore set forth together with many thoroughly practical advanta'gesare successfully achieved. As many possible embodiments" m'aybe made of the above invention and as many changes might be'made in the embodiments above'set forth, it is to-be-understood that all matter hereinbefore set'forth or shown in the accompanying drawings is to be interpreted"as"illustrative'andnot'in a limiting sense.

We. claim:

1i A' method-for eroding stock from a workpiece which comprises contacting a' workpiece with the shaped suffaceofamolded body of carbide selected from the group consisting of self-bonded boron carbide, recrystallized silicon carbideand mixtures thereof, moving the workpiece relative to said surface while maintaining contact thereb'etween, applying an electrolytic liquid" to said surface, and flowing a direct electric current across the interface between said workpiece and said surface through the electrolytic liquid with said workpiece as an anode and' saidsurf'aceas a cathode.

2. The method of claim 1, wherein said'carbide of'said' molded body is a mixture of 35 to 95 by weightsilicon carbide=and"5 to 65% by weight boron carbide.

3. A method of electrolytic eroding whichcomp'ri'ses contacting-a workpiece with a rotating surface of an eroding wheel of carbide selected from the group-consistingof-self-bonded boron carbide, recrystallized-silicon carbide and mixtures thereof, applying an electrolytic liquid to the eroding. wheel and passing adirect electric currentfirom the-workpiece through said electrolytic liquidEto-the'erodin'giwheebso-asto make the workpiece an anode andthe wheel a cathode.

4;. The-methodof claim 3wherein said carbideofssaid eroding-wheel is a mixtur'e' of' 35 to;95% by weight siliconcarbide'andd to-65% by'weight boron'carbidel- 5. ALmet-hod ,forremoving stock-from -a-'workpiece: by electrolytic action which comprisesfiowing directeleetrie current. through electrolytic -liquid 'between the workpiece asianode and-a rotating eroding wheel of'carbide'selected from-jtheggroupjconsistiim of self-bondedwboromcarlide;

I2 recrystallized silicon carbide and mixtures thereof as cathode.

6; The-method of claim 5, wherein said carbide of-s'aideroding wheel is a mixture of 35-to by weight silicon carbide and 5 to 65% by weight boron carbide.

7. A method'for removing stock from a workpieceby electrolytic-- action inaccordance with claim 5 which also comprises flowing direct electric current through the elec-' trolytic'liquidbetween'the rotating eroding' wheel as anode and an electrically conductive piece as cathode whereby also to remove particles from the workpiece collecting on the eroding wheel.

8. A method for removing stock from a workpiece b'y" electrolytic action in accordance with claim 6' which also" comprises flowing direct-electric current through the elec-" trolytic' liquid from the rotating eroding wheel as anodeto an electrically conductive block as cathode whereby also to remove particles from the workpiece collecting" on the eroding wheel;

9. A methodfor removing stock froma workpiece which comprises contactinga workpiece with" the shapedsurface of anintegralbody of recrystallized silicon bide, moving the-workpiece'relativeto saidsurface while" maintaining contact therebetween,applying an electrolytic liquid to said surfacej-an'd flowing a direct electric-current across the interface be'tween said workpiece and said surface through the electrolytic liquid with said workpiece as'ah-aiiode andsaid surface as a cathode l0.'- Amethod of electrolytic eroding wbi fie contacting "a' workpiece with a" rotating surface of an eroding wheel' of integral recry'stallizedwili'com carbide,= applying an electrolytic liquid to theeroding"wheel' aild passing a direct electric current from the wo k iece th'rbugh said electrolytic liquid to 'the"e"rodir'1g wheel so as to" make the-workpiecean anode and the wheel a: cathode;

1-1; A method-for removing stock from workpiece" 12. A method for removing stock from a workpiece by electrolytic actio'n'in" accordance with claim 11 which also comprises flowing direct electric'current throu'gh the" electrolytic liquid from the rotating eroding wheel as anode to an electrically conductive block as" cathode whereby also to remove particles" from theworkpiece collecting'o'n' the eroding wheel! 13. A method for" eroding stock from a workpiece which comprisescontacting a workpiece with the shaped surface of a molded body of self-bonded b'oron carbide, moving the workpiece-relative to said surface while main taiuing contact therebetween, applying an electrolytic liquid to said'surface, and flowing a direct electric current across the interface between said workpiece and said surface through theelectrolytic-liquid with said workpiece as an anode and said surface as a cathode. v

14. A method of electrolytic eroding which comprises contacting. a workpiece with a rotating surface of an eroding'wheel of self-bonded-boron carbide, applyingan electrolytic liquidto the eroding wheel and passing a direct electric current from the workpiece through said electrolytic liquid 'to the eroding wheel so as-to make the workpiece an anode and the wheel a-cathode.

15'. A method for removing stock-from a workpiece by electrolytic action-which comprises flowing direct electric current through electrolytic liquid between the workpiece as an anodeand a rotating eroding wheel of self-bonded boron carbide as a cathode.

16. A method for removing stock from a workpieceby electrolytic action in accordance with claim 15 which also comprises flowing direct electric currentthrough the electrolytic liquidfrom the rotating-.eroding'wheef as -anode -tman; electrically conductive bloclc-as cathode wherebyalso to remove particles from the workpiece collecting on the eroding wheel.

References Cited in the file of this patent UNITED STATES PATENTS Fitzgerald May 22, 1900 Walker Mar. 26, 1901 Jenkins Feb. 20, 1912 Hyde et a1 Mar. 30, 1915 Ridgeway Feb. 14, 1933 Johnston Nov. 5, 1935 14 2,027,786 Ridgeway Jan. 14, 1936 2,359,920 Keeleric Oct. 10, 1944 FOREIGN PATENTS 1,094,600 France Dec. 8, 1954 OTHER REFERENCES Steel, vol. 130, No. 3, Mar. 17, 1952, pp. 84-86.

American Machinist, Sept. 28, 1953, pp. 122-125.

J. Electrochem. Soc., vol. 100, May 1953, pp. 1250 and 1260. 

1. A METHOD FOR ERODING STOCK FROM A WORKPIECE WHICH COMPRISES CONTACTING A WORKPIECE WITH THE SHAPED FACE OF A MOLDED BODY OF CARBIDE SELECTED FROM THE GROUP CONSISTING OF SELF-BONDED BORON CARBIDE, RECRYSTALLIZED SILICON CARBIDE AND MIXTURES THEREOF, MOVING THE WORKPIECE RELATIVE TO SAID SURFACE WHILE MAINTAINING CONTACT THEREBETWEEN APPLYING AN ELECTROLYIC LIQUID TO SAID SURFACE, AND FLOWING A DIRECT ELECTRIC CURRENT ACROSS THE INTERFACE BETWEEN SAID WORKPIECE AND SAID SURFACE THROUGH THE ELECTROLYTIC LIQUID WITH SAID WORKPIECE AS AN ANODE AND SAID SURFACE AS A CATHODE. 